1. Field of the Invention
This invention relates to regulation of cell growth and proliferation and specifically to a novel cell cycle-related polynucleotide, 5'ALT, and novel polynucleotides encoding truncated cell cyclin inhibitors, p16.sup.INK4A and p15.sup.INK4B.
2. Description of Related Art
The growth cycle of eukaryotic cells is regulated by a family of protein kinases known as the cyclin-dependent kinases ("CDKs"). The cyclins and their associated CDKs move cells through the three phases of the growth cycle (G1, S and G2, respectively) leading to division in the mitosis phase (M). The cyclin/CDK complexes whose role in cellular proliferation has been most clearly defined to date are the cyclin D/CDK enzymes, which are believed to assist in the progression of the G1 growth cycle phase. Of these enzymes, cyclin D1 is believed to be an oncogene, whose overexpression stimulates excessive cell division through the continuous production of kinase, thus contributing to the development of cancers of, for example, the breast and esophagus. Cyclin D1 is specifically bound by CDK4 as part of a multi-protein complex that also consists of a protein known as p21 and cell nuclear antigen.
Known inhibitors of such cyclin/CDK overexpression include the tumor suppressor protein p53 and the protein product of the retinoblastoma (Rb) gene. Recently, two putative inhibitors of cell cyclins, p16.sup.INK4A and p15.sup.INK4B, were isolated (Serrano, et al., Nature, 366:704, 1993; Hannon, et al., Nature, 371:257, 1994, respectively).
The cyclin-CDK inhibitors p16.sup.INK4A (CDKN2/MTS-1) and p15.sup.INK4B (MTS-2) are important components of cell cycle regulation. Transition through G1 is promoted by the cyclin-dependent protein kinases CDK4 and CDK6 which phosphorylate Rb resulting in release of E2F and cell cycle progression (Hunter, T. & Pines, J. ,Cell 79: 573-582, 1994). In addition to more universal inhibitors (Morgan, D. O., Nature, 374:131-134, 1995), these kinases are strongly inhibited by both p16.sup.INK4A and p15.sup.INK4B . Isolation of the genes for these negative cell cycle regulators was quickly followed by their co-localization to chromosome 9p21, within a critical region commonly deleted in many types of human cancer (Kamb, A., et al., Science, 264:436-440, 1994; Nobori, T., et al., Nature, 368:753-756, 1994). Familial and sporadic malignant melanomas have been consistently associated with cytogenetic abnormalities of chromosome 9p21 (Fountain, et al, Proc. Natl. Acad. Sci., USA, 89:10557, 1992; Cannon-Albright, et al., Science, 258:1148, 1992). Deletions of this region are also common in gliomas (Olopade, et al., Cancer Res., 52:2523, 1992), lung cancers (Olopade, et al., Cancer Res., 53:2410, 1993), and leukemias (Olopade, et al., Genomics, 14:437, 1992). Although excellent tumor suppressor gene candidates, somatic point mutations were found to be rare in many primary human tumors with hemizygous loss of 9p21 (Cairns, et al. , Science 245:415-416, 1994).
If the cyclins are overproduced in a cell or made at an inappropriate time, they would be expected to stimulate inappropriate cell division by keeping their partner kinases "on" when they should be turned off, a malfunction that could lead to cancer or otherwise unwanted cellular proliferation. There remains a need to identify the control signals which determine whether a cell proliferates or not.